The present invention relates to formulations of clomazone having reduced volatility relative to conventional emulsifiable concentrates of clomazone. In particular it relates to microencapsulated formulations of clomazone in which the clomazone is encapsulated in a shell of polyurea.
Clomazone, the common name for 2-(2-chlorophenyl) methyl-4,4-dimethyl-3-isoxazolinone, a highly effective herbicide, is also highly volatile, so much so that clomazone applied to the soil in a target area may move to adjacent area s and there cause discoloration, most typically whitening or some degree of bleaching, of a variety of crops, trees, or decorative plants. While this bleaching, indicative of the mode of action of the herbicide, may be temporary when plants are exposed to sufficiently low concentrations, it is unwelcome, even when it does not result in the destruction of the plant. Accordingly, the label for the use of Command(copyright) 4 EC Herbicide, an emulsifiable concentrate formulation in commercial use that contains four pounds of clomazone per gallon of formulation, lists a number of restrictions on how the product is to be used, including weather conditions, spray volume and pressure, and distance from areas where plants are in commercial production. For example, for preemergent applications clomazone is not to be applied within 1,500 feet of commercial fruit, nut, or vegetable production or commercial greenhouses or nurseries. Clearly, this is a severe limitation on the use of an herbicide.
It is the purpose of the present invention to reduce the volatility of clomazone formulations, so that problem of off-site injury is significantly reduced, i.e., by at least fifty percent, while maintaining a satisfactory level of herbicidal activity in the target area.
Attempts to prepare formulations of encapsulated clomazone by the general methods known to the art, including polyamide shells as well as polyurea, frequently resulted in formulations that not only gave little or no reduction in volatility, but had poor physical characteristics, e.g., undesirable agglomeration of the capsules or separation of phases. Perhaps one factor accounting for the difficulty in preparing satisfactory formulations is the significant water solubility of clomazone. No reports of formulations of encapsulated clomazone have been found.
It has now been found that encapsulated formulations of clomazone for which the volatility is reduced to fifty percent or less than that of the commercially available Command(copyright) 4 EC emulsifiable concentrate of clomazone, and which retain a satisfactory level of herbicidal activity, can be prepared, provided that the isocyanate and amine moieties that are to form the polyurea shell wall are carefully selected.
The process of the invention involves the following steps: (a) providing an aqueous phase containing an emulsifier, preferably a partially hydrolyzed polyvinyl alcohol; an antifoam agent, and optionally a xanthan gum viscosity modifier/stabilizer; (b) providing a water immiscible phase consisting of clomazone and polymethylene polyphenyl isocyanate, with or without a hydrocarbon solvent; (c) emulsifying the water immiscible phase in the aqueous phase to form a dispersion of water-immiscible droplets throughout the aqueous phase; (d) agitating the dispersion while adding to it, either neat or in aqueous solution, ethylenediamine, diethyltriamine, triethylenetetramine, 1,6-hexanediamine, or a mixture of the polyfunctional amines, thus forming a polyurea shell wall around the water-immiscible droplets. Once the microcapsules are formed, the suspension is cured by moderate heating, after which one or more stabilizing agents, such as propylene glycol, xanthan gum, smectite clay, or an ionic dispersing agent such as a sulfonate of an alkyl napthalene, may be added, as is well-known in the art. It has also been found that adjusting the pH of the formulation from mildly acidic to mildly alkaline conditions, such as a range of from 6.5 to 9.0, e.g., pH 8.9, results in a formulation having improved storage stability. The addition of these materials after encapsulation and curing to adjust viscosity and suspensibility is not seen to have any effect on the loss of clomazone through volatility or on the herbicidal efficacy of the formulation.
The aqueous phase will ordinarily contain 0.3 to 3.0, preferably 0.8 to 2.0, weight percent of one or more emulsifiers, e.g., polyvinyl alcohol, 0.05 to 0.20, preferably 0.06 to 0.15, weight percent of the xanthan gum viscosity modifier/stabilizer, if it is used, and 0.1 to 1.0, preferably 0.4 to 0.9, weight percent of the antifoam agent.
The water-immiscible phase will ordinarily consist of 60 to 85, preferably 65 to 77, weight percent of clomazone, an amount of polymethylene polyphenyl isocyanate (PMPPI) such that the ratio of clomazone to PMPPI is in the range of 1:1 to 6:1, preferably 4.5:1 to 4.8:1, and an aromatic hydrocarbon solvent for the two solutes. However, use of solvent is optional in the preparation of formulations containing more than about two pounds of clomazone per gallon of formulation. In such preparations a small amount of solvent may still be used to depress the melting point.
The amine solution will ordinarily contain 10 to 100, preferably 30 to 40, weight percent of ethylenediamine, diethylenetriamine, triethylenetetramine, 1,6-hexanediamine, or preferably a mixture of the polyfunctional amines, with ethylene diamine being used only in a mixture.
The emulsification step requires high shear mixing to give small droplets of the immiscible phase. Factors that influence droplet size, which determines the eventual size of the microcapsules, as well as the stability of the emulsion, include speed and length of mixing, the type and amount of surfactant, solvent, temperature, and viscosity, as well as the xanthan gum, when used. Selection of the appropriate microcapsule size to achieve the purposes of the invention requires a balance between competing factors. In general, increasing microcapsule size decreases volatility, but also decreases suspensibility of the particles, while decreasing size yields better suspensibility, but higher volatility. For the purposes of the present invention the average size of the microcapsules is 5 to 50 microns, preferably 5 to 30 microns. The operating conditions to yield microcapsules of a desired size will depend on the emulsifying equipment used, and the adjustment to determine the proper conditions is well within the skill of the art.
In contrast to the conditions of the emulsification step, agitation during the amine addition should be gentle. Stirring is continued while the suspension is cured by heating to a temperature of 35 to 60, preferably 45xc2x0 to 50xc2x0 C., for 3 to 10, preferably 4 to 5, hours.
The amounts of post encapsulation additives to be added typically would be selected from one or more of 0.75 to 6.5 wt. % propylene glycol, 0.05 to 0.30 wt. % xanthan gum, 0.25 to 0.50 wt. % smectite clay, and 0.5 to 6.0 wt. % one or more surfactants, each weight percent relative to the weight of the formulation after addition of the stabilizers.
The formulations of the present invention are prepared by the methods exemplified in the following examples.